Fused optical fiber components are nowadays very common in fiber laser, optical communications and other optical fiber based systems. The main process for manufacturing such a component is fusion of the fibers material in order to manipulate their shape. Usually, the heat for the fusion is generated by means of hydrogen flame, arc discharge, filament or laser beam. However, all of these methods require tightly controlling the amount of heat that is absorbed in the fibers for the process to be precise and reproducible. Controlling of heat absorption can be performed by measuring the temperature of the heated fibers section, which is typically 1-3 mm in length and 0.01-1 mm in diameter. Direct measurement of the temperature under such circumstances is complicated, since measurement by contact will disturb the fusion process. Another problem is that it is very difficult to distinguish between the fiber temperature and the temperature of the heat source itself without touching the fiber.